Mimir, Old Norse Mímir, in Norse mythology, the wisest of the gods of the tribe Aesir; he was also believed to be a water spirit. Mimir was sent by the Aesir as a hostage to the rival gods (the Vanir), but he was decapitated and his head was returned to the Aesir. The god Odin preserved the head in herbs and gained knowledge from it. According to another story, Mimir resided by a well that stood beneath one of the roots of Yggdrasill, the world tree. That well, sometimes called Mímisbrunnr, contained one of Odin’s eyes, which Odin had pledged in order to drink from the waters and receive wisdom.